Cutting tools are commonly used to perform metalworking processes such as drilling, turning, forming, sawing, grinding, milling, tapping, threading, honing, broaching, and similar machining operations. Metalworking process efficiency is highly dependent on proper lubrication and cooling of both the cutting tool and a work piece.
Many different types of cutting fluids, or coolants, have been designed to meet the lubrication and cooling requirements of diverse situations. The most widely used metalworking coolants are soluble oils. Such soluble oils are typically an emulsion of a soluble mineral oil and an emulsifier such as soap and water, in water to oil proportions ranging from 100:1 to 5:1. They enjoy popularity because they combine the lubricating abilities of oil with the better cooling abilities of water, and are relatively easy to manufacture.
Most machine tools recirculate coolant during their operation. Coolant contamination is inevitable, however, so that at some point the coolant must be changed. Contamination originates from many sources. Fine metal particles and detritus from the metalworking operations accumulates as abrasive sludge. Dirt and rust, as well as lubricants and hydraulic fluids from the machine tools themselves (tramp oil), invariably find their way into the coolant. Bacteria and fungi often flourish in oil-water mixtures, giving rise to the formation of corrosive acids. The result is not only offensive odors, but also significantly reduced coolant efficiency. The coolant can be filtered to remove metal particles and skimmed to remove sludge, but at some point the coolant simply is no longer effective, and must be thrown away.
In the not too distant past, the spent coolant could be disposed of by simply bringing it to the local landfill. As society has come to realize, careless waste disposal often creates a major health hazard. Oily industrial waste waters such as spent coolant are now considered by many environmental regulatory agencies as hazardous material. They can be shipped only to specified hazardous dump sites which may be located many miles from where the waste is generated. Careful disposal record-keeping must be maintained -- in some locations, expensive chemical analysis on each shipment is even required. The resulting transportation, fees, taxes, and administrative expenses mean that the disposal cost of coolant often exceeds its original purchase price.
The cost of coolant disposal thus increases the total cost of a given metalworking operation. In fact, the cost of waste coolant disposal now has a significant impact on the profit margin of small metalworking shops which typically produce less than one thousand or so gallons of waste fluid per month. In that environment, spent coolant is typically handled by first collecting it in fifty-five gallon drums. The drums are then periodically shipped to an approved waste site for disposal.
There have been prior attempts to minimize the cost of waste disposal. As previously mentioned, most cutting coolant is as much as ninety percent water by volume. For every ten drums or so of contaminated coolant, then, there really is only one drum of hazardous material. It is advantageous if the contaminated coolant can be first processed into a concentrate. A mobile system for cleaning and recovering coolant is described in U.S. Pat. No. 4,636,317. That system includes a solid separation filter, a sterilizer for removing bacteria by pasteurization or radiation, a disk bowl centrifuge, and storage tanks for holding extracted contaminants. It is physically large enough to require a truck for transport.
The purchase price of such treatment equipment, however, is in the tens of thousands of dollars. This is simply too expensive for small metalworking shops to purchase. As one alternative to shipping the untreated coolant, they can hire or lease a mobile recovery system to periodically process the waste into concentrate on the shop premises. They then need to ship only the concentrated fluid to the approved waste site. However, even the cost of hiring a recovery system and shipping the processed waste tends to make it difficult for small metalworking shops to price their services competitively with larger shops.
So-called ultra-filtration systems are also known which use a semi-permeable membrane as a filter to separate contaminants. While ultra-filtration technology has been found useful in high-volume applications, its application until now in low-volume applications has also been limited because of high cost and the difficulty in applying this technology to treat waste water stored in drums.